Dissertations and Theses - Physics
http://hdl.handle.net/2142/8859
Dissertations in PhysicsMon, 21 May 2018 15:10:03 GMT2018-05-21T15:10:03ZImaging vortex dynamics in Josephson arrays using magnetic force microscopy
http://hdl.handle.net/2142/98326
Imaging vortex dynamics in Josephson arrays using magnetic force microscopy
Naibert, Tyler R
Vortices and vortex lattices play a major role in determining the transport properties of type-II superconductors[1–3], and enable a platform to investigate exotic superconducting physics[4,5]. The study of vortex matter has generally focused on novel states in 2D ﬁlms and structures, and has recently moved to investigating systems with constrained dimensions and smaller vortex numbers[6–11]. Vortices are responsible, for example, for some electrical transport regimes in superconducting ﬁlms, as well as the Berezinkskii-Kosterlitz-Thouless phase transition in superconducting ﬁlms[12]. Unconventional forms of superconductivity, such as the spin triplet pairing predicted in Sr2RuO4, or in topological insulators paired to s-wave superconductors, contain two condensates that may support two vortex lattices, and may display Majorana modes, signatures of which may have been seen in other superconducting systems[13–17]. The vortex-vortex interactions, or inter and intra-condensate couplings in multicondensate systems, are important parameters that characterize the behavior of the systems that display such phenomena[18–20]. In investigating these parameters, a technique that can both probe the energies in a system, as well as manipulate the vortices therein, has long been desired.
In this work, we report on progress in determining the energy scales of vortex systems, as well as limited control over the vortex motion. Using a technique based on magnetic force microscopy, we can directly measure the resonant motion of vortices present in a superconducting lattice. We use a scanning magnetic tip to trap a small number of vortices in a superconducting Josephson junction array near the tip. By observing the resonant motion of the conﬁguration of vortices, a map of the location of energy degeneracies between diﬀerent stable conﬁgurations is generated. From this data, we use a simulation to extract the relative strengths of the characteristic energy scales for the system, including the vortex-magnetic ﬁeld interaction, the vortex-vortex interaction strength, and the chemical potential for the vortices. The simulations for small numbers of vortices ﬁts the data well for multiple ﬁeld proﬁles and lattice spacings. The ability to tune the vortex number and conﬁgurations by changing the magnetic ﬁeld proﬁle from the tip, as well as the lattice parameters of the superconducting surface, are key portions of this technique. We demonstrate that the relative strengths of the chemical potential and vortex-vortex interactions can be tuned relative to the vortex-magnetic ﬁeld energy by changing the lattice spacing of the array. We also show that by moving the tip farther from or closer to the surface, which changes the potential well from the tip, that the conﬁgurations of vortices can be modiﬁed. From the experiments, we show that this technique can be used to both extract the strengths of the relative energy scales in this system and other superconducting systems, as well as for manipulating the vortex conﬁgurations for quantum computation applications.
Magnetic force microscopy; Superconductor; Superconductivity; Vortex; Josephson junction; Josephson junction array (JJA); Superconductor-normal metal-superconductor (SNS) array; Vortex interactions; Pinning
Mon, 03 Jul 2017 00:00:00 GMThttp://hdl.handle.net/2142/983262017-07-03T00:00:00ZNaibert, Tyler RAspects of quantum entanglement in critical and topologically ordered systems
http://hdl.handle.net/2142/98200
Aspects of quantum entanglement in critical and topologically ordered systems
Wen, Xueda
Quantum entanglement plays an important role in characterizing the property of many-body systems and quantum field theories. In this thesis, we study the quantum entanglement in (1+1)-dimensional critical systems and (2+1)-dimensional topologically ordered phases. For (1+1)-d critical systems, we mainly study the non-equilibrium property of quantum entanglement, by focusing on three interesting cases: (i) the time evolution of entanglement hamiltonian during thermalization of a subsystem after a global quantum quench; (ii) time evolution of entanglement entropy after an inhomogeneous quantum quench; (iii) entanglement negativity evolution after a local quantum quench. For (2+1)-d topologically ordered phases, we use edge theory approach to study the topological entanglement entropy, mutual information and entnanglement negativity of Chern-Simons theories, which are further confirmed based on the surgey approach. In addition, we study the entanglement renormalization of topological insulators, and investigate the geometric and topological properties in the bulk of entanglement renormalization.
Entanglement; Critical systems; Topologically ordered systems
Thu, 06 Jul 2017 00:00:00 GMThttp://hdl.handle.net/2142/982002017-07-06T00:00:00ZWen, XuedaSymmetry-protected topological phases and quantum anomalies
http://hdl.handle.net/2142/98272
Symmetry-protected topological phases and quantum anomalies
Hsieh, Chang-Tse
We present the correspondence between symmetry-protected topological (SPT) phases and their anomalous boundary states, based on examples in various spacetime dimensions. Through the study of the effect of interactions on these SPT phases, we discovered a new formalism of quantum anomalies, associated with discrete spacetime (such as time-reversal and spatial reflection) symmetries in particular, to classify distinct interacting topological phases. An example is the Z2 classification of the (2+1)d topological insulator protected by charge U(1) and time-reversal (or CP) symmetries, which can be deduced by the form of the global U(1) gauge anomalies on its edge theories defined on closed unorientable manifolds. In this case, the nontrivial phase (in free systems) is robust against electron interactions. Another example is the (3+1)d topological superconductor protected by only time-reversal or reflection symmetry. For this system, we identified the bulk phase by studying the global gravitational anomalies of the surface theories formulated on unorientable spacetime manifolds, and also discussed its connection to the collapse of the non-interacting classification by an integer Z to Z16, in the presence of interactions.
We also revisit the problem of gauging a discrete internal symmetry in theories of chiral (Weyl) fermions in 3+1 dimensions – which have not been fully understood so far – from the perspective of fermionic SPT phases in 4+1 dimensions. Comparing with the previous results, we give a complete answer for the anomalies constraints on the discrete symmetry, as our approach is based on purely geometrical considerations, namely, our assumption is more fundamental and general. Furthermore, our result also provides an understanding of gapped states of fermions with anomalous discrete symmetries, and we present a model, based on weak coupling, for constructing these anomalous gapped states.
Symmetry-protected topological phases; Topological insulators and superconductors; Classification; Strongly correlated systems; Quantum anomalies; Orientifold conformal field theories; Discrete gauge anomalies; 't Hooft anomalies
Tue, 11 Jul 2017 00:00:00 GMThttp://hdl.handle.net/2142/982722017-07-11T00:00:00ZHsieh, Chang-TseGeometry and topological phase of matter
http://hdl.handle.net/2142/98214
Geometry and topological phase of matter
You, Yizhi
In this thesis, I will first derive and study the effective field theories of isotropic-nematic quantum phase transitions of Quantum states. The low-energy theory of the nematic field has z=2 dynamics due to a Berry phase of the order parameter, which is related to the Hall viscosity in parity and time-reversal-symmetry (TRS) broken states. The vortex of the nematic field, which is physically a disclination, creates a nonzero geometry curvature in the disclination core. The leading coupling between the nematic field and gauge field includes a Wen-Zee term which links the geometry curvature with the gauge theory.
In the second part of this thesis, I investigate the geometry related issues in Weyl semimetals and SPT states, and explore the novel character of geometry defect in SPT states inherited from the topological nature of manybody system. In addition, I would introduce a general way to induce topological phase transition via decorated defect condensate.
In the final part of this thesis, I begin with the
bilayer Half-filled Landau Level system where the two composite Fermi surface
acquires interlayer coherence and forms bonding/anti-bonding composite fermi
sea. The corresponding interlayer coherent composite Fermi liquid(ICCFL) phase
provides a straightforward landscape to verify the Dirac nature in Son's theory
and extract the hidden Berry phase structure of the composite Fermi surface.
The ICCFL phase contains two Fermi surfaces which are detached in most regions
but adhesive at two hot spots. Such nematic structure is a consequence of the Berry phase encoded in the Dirac Fermi surface which is absent in HLR theory.
Due to the nematicity in ICCFL, the system supports half-quantum vortex with
deconfined $\frac{\pi}{2}$ gauge flux and the phase transition toward ICCFL
contains a Lifshitz criticality with $z=3$ dynamical exponent. In addition, the
exciton order parameter carries topological spin number so the ICCFL contains a
unique Wen-Zee term which connects EM response with the background geometry
curvature.
Topological; Geometry
Fri, 19 May 2017 00:00:00 GMThttp://hdl.handle.net/2142/982142017-05-19T00:00:00ZYou, YizhiSelective molecular transport in nanopore systems
http://hdl.handle.net/2142/98107
Selective molecular transport in nanopore systems
Decker, Karl Steven
A detailed characterization of the physics of novel nanopore systems has the potential to revolutionize water filtration, nanofluidics, and biomolecule detection technologies. I give my characterizations of five nanopore systems as my dissertation. First, I present my study of nanopores in polyethylene terephthalate (PET), revealing the mechanism for variance in current rectification based on cation species. Second, I demonstrate the mechanism of selective probe capture in bacterial toxin protein α-hemolysin (aHL) using dielectrophoresis. Third, I introduce the first simulation of molecular artificial water channel pillar[5]arene (PAP) and uncover the mechanics of its water transport and self-aggregation properties. Fourth, I characterize the water permeability and ion rejection of truncated human membrane protein aquaporin-1 (AQP) in simulation. Finally, I present MD simulation of truncated AQP as a voltage-gated ionic diode and as the functional element of a double-membrane ionic pump.
Molecular transport; Nanopore; Biomolecule; Ion current; Filtration
Fri, 02 Jun 2017 00:00:00 GMThttp://hdl.handle.net/2142/981072017-06-02T00:00:00ZDecker, Karl StevenCosmological signatures of fundamental physics
http://hdl.handle.net/2142/98101
Cosmological signatures of fundamental physics
Banerjee, Arka
This thesis deals with the study of the cosmological signatures of certain aspects of fundamental physics, and how cosmological observables can be used to constrain properties of fundamental particles. Over the past decades, increasingly precise measurements in cosmology have become powerful probes of fundamental physics - for example, the inference of dark matter and dark energy from cosmological observations remain the most significant evidence of new physics beyond the Standard Model of particle physics. Another example is the cosmological scalar-to-tensor ratio, which can potentially differentiate different models of inflation and other early Universe theories.
Determining the absolute mass scale of neutrinos is an interesting problem in particle physics, and can shed light on the mass generation mechanism for neutrinos, which, in turn, can tell us about physics beyond the Standard Model. To fully exploit the signatures of massive neutrinos on cosmological observables, one needs to perform accurate simulations. In this thesis, we explore a new method for performing neutrino simulations, which overcome the shortcomings of previous methods which were employed. From these simulations, we identify an observable which is very sensitive to the neutrino mass - the clustering of cosmological voids on large scales. We also forecast how well the neutrino masses and thermal ``dark radiation'' models can be constrained in future cosmological surveys using their effect on various observables in these surveys, such as the clustering of galaxies, galaxy-galaxy lensing, and cosmic shear.
Cosmological observables can also be used to constrain the properties of Dark Matter itself. While Dark Matter has traditionally been considered a collisionless fluid, there has been recent interest in self-interactions of dark matter. We consider a special form of self-interactions in this thesis - where the interactions are elastic but anisotropic. We develop the formalism and methods to simulate these interactions, and study the signatures of these interactions on the properties of dark matter halos.
Cosmology; Large-scale structures; Neutrinos; Galaxy surveys; Self-interacting dark matter
Fri, 19 May 2017 00:00:00 GMThttp://hdl.handle.net/2142/981012017-05-19T00:00:00ZBanerjee, ArkaPhase transitions and vortex dynamics in superconducting island arrays
http://hdl.handle.net/2142/98090
Phase transitions and vortex dynamics in superconducting island arrays
Durkin, Malcolm Shaw
In this thesis, we use superconducting island arrays as a platform for studying vortex motion and quantum phase transitions. We investigate superconducting vortex dynamics and lattice structures in superconducting arrays by performing electrical transport measurements on Nb island arrays on Au at milli-kelvin temperatures and finite fields. At low fillings, we observe anomalous vortex dynamics that we attribute to a history dependent dissipative force as the vortex moves through the lattice. At higher fillings, vortex-vortex interaction becomes significant and is dominated by collective vortex motion. We find that the transition from pinned to vortex lattice flow is split into two transitions as the filling is shifted from the commensurate filling regime, where the vortex lattice has strong crystalline order, to an incommensurate filling, where the vortex lattice no longer matches potential wells of the SNS array. We find that this behavior is consistent with domain wall motion in a polycrystalline vortex lattice at commensurate fillings.
Superconducting island arrays can also be used to study phase transitions. Previous work in our group found that the onset of superconductivity in Nb islands was strongly dependent on the island spacing in the array. Performing follow up measurements, we find that the critical island temperature increases as the underlying Au is made thicker, indicating that this effect is dependent on the strength of electrical interactions between islands and is not due to normal metal suppression. Performing measurements on individual islands, we find that the vast majority of 260nm islands undergo a transition at temperatures far lower than those in island arrays, to the extent that they cannot be observed in a Helium 4 cryostat, and that there is a broad distribution of island critical temperatures observed. This suggests that the onset of superconductivity in rare ordered regions plays a significant role the onset of superconductivity in both the arrays samples and single islands. Lastly, we present work studying the superconductor to insulator transition in Sn island arrays on graphene as well as the technical difficulties involved.
Superconductivity; Vortex; Condensed matter; Dynamics; Defect; Glass
Tue, 02 May 2017 00:00:00 GMThttp://hdl.handle.net/2142/980902017-05-02T00:00:00ZDurkin, Malcolm ShawReal-space magnetic imaging of the spinel MnV2O4
http://hdl.handle.net/2142/98089
Real-space magnetic imaging of the spinel MnV2O4
Wolin, Brian Augustus
Controlling multiferroic behavior in materials will enable the development of a wide variety of technological applications. However, the exact mechanisms by which multiferroic behavior arises in many materials are not well understood. One such class of materials are the spinels, including MnV2O4. Bulk probe studies of this compound have yielded conflicting and inconclusive results. To inform the debate and better understand the underlying physics, we performed magnetic force microscopy measurements on two MnV2O4 samples with differing degrees of mechanical strain. These local investigations revealed sub-um-scale patterns in the magnetic structure in both samples. In one case, the magnetization of these stripes is estimated at Mz ~ 10^5 A/m, which is on the order of previous saturation magnetization measurements.
The discovery of such large inhomogeneities necessitates revision of theoretical proposals and reinterpretation of experimental data regarding the low-temperature phases of the spinels. Similar MFM measurements of a related material (Mn3O4) provide evidence that magnetic inhomogeneity is a common feature in the magnetically ordered phases of multiferroic spinel compounds.
Spinel; Spinels; Multiferroic; multiferroics; multiferroism; Magnetic force microscopy (MFM); MnV2O4; Magnetic imaging; Strain; Quantitative magnetic force microscopy (MFM)
Mon, 01 May 2017 00:00:00 GMThttp://hdl.handle.net/2142/980892017-05-01T00:00:00ZWolin, Brian AugustusScanning tunneling microscopy and spectroscopy of topological materials with broken symmetry
http://hdl.handle.net/2142/97772
Scanning tunneling microscopy and spectroscopy of topological materials with broken symmetry
Scipioni, Kane Lee
This dissertation focuses on the probing of physics governing the electronic and structural properties of topological materials. In three-dimensional topological insulators, the native substitutional defects result in a shift of the chemical potential into the conduction and valence bands. The added conduction channels obscure the physics of the topological surface states. Chemical tuning has been used previously to counteract this parasitic conductivity. However, many details of the process are not well understood and the conditions required to produce optimal samples are not yet well established. In the first project, scanning tunneling spectroscopy was used to observe Landau quantization in thin films of (Bi1-xSbx)2Te3. By combining nanoscale imaging and spectroscopy, the sensitivity of the chemical potential to the chemical composition and thin film growth conditions was studied. The results demonstrate the multi-dimensional parameter space required to obtain an intrinsic topological insulator and provide knowledge to optimize the electronic properties of topological materials. Magnetic doping was then added to induce ferromagnetism, which creates an energy gap in the surface states of (Bi1-xSbx)2Te3. Tunneling conductance spectroscopy was used to examine the correlation between the density of magnetic impurities and the size of the surface band gap. The results indicate that large concentrations of Cr create impurity states inside the gap that reduce the effective gap magnitude. Finally, Landau level spectroscopy was applied to the surface state of Pb1-xSnxSe where the signature of electron-phonon coupling was extracted from the surface state dispersion and was used to determine the mass enhancement factor.
Molecular beam epitaxy; Thin film; Scanning tunneling microscopy; Topological insulator; Quantum anomalous hall effect; Electron-phonon coupling
Fri, 21 Apr 2017 00:00:00 GMThttp://hdl.handle.net/2142/977722017-04-21T00:00:00ZScipioni, Kane LeeCharacterization of collective phenomena in cuprate superconductors by momentum-resolved electron energy loss spectroscopy
http://hdl.handle.net/2142/97661
Characterization of collective phenomena in cuprate superconductors by momentum-resolved electron energy loss spectroscopy
Vig, Sean D
There exists a wide variety of strongly correlated electronic materials, including high temperature superconductors, exotic topological insulators, and charge density wave materials, that exhibit emergent behavior that cannot be understood through current theories of metals and insulators. These materials are classified by the low temperature ordered phases these materials take on, and there may be several ordered phases interacting over the various regions of phase space. High temperature superconductors, for example, may also exhibit Mott insulator, charge density wave, spin glass, and pseudogap phases in their phase diagram. The current consensus in describing most strongly correlated electron systems is, at best, qualitative, and understanding these ordered phases, the mechanisms that drive them, and their relation to related emergent phenomena is necessary for developing a microscopic understanding of this class of materials.
The electron-electron interactions that drive the collective electronic state can be described by the dynamic charge susceptibility, χ(q,ω). This quantity encodes information about the propagation of density fluctuations in a system—the collective "sloshing" of electrons—as mediated by bosonic excitations. In order to measure this quantity, we have developed a new experimental technique, momentum-resolved electron energy loss spectroscopy (M-EELS). As a part of this development, we derived the theoretical framework for describing the scattering cross section in terms of the imaginary part of the dynamic susceptibility, χ''(q,ω), and implemented the software and hardware stack necessary to perform this measurement. Using this technique, we are able to measure finite momentum charge dynamics at 2 meV energy scales.
We used M-EELS to investigate materials in the Bi2Sr2CaCu2O8+δ (Bi2212) family of cuprate high temperature superconductors. In studying optimally doped crystals, we have performed a full energy and momentum characterization of the low-energy dynamic susceptibility. Our measurement identifies known phonon modes, as previously seen in Raman scattering, infrared spectroscopy, and HR-EELS. Using a one-loop correction model, we have identified these modes as giving rise to the kinks in the electron spectral function as seen in ARPES, suggesting that these modes are related to the emergent phenomena exhibited in these materials. Furthermore, we observe a background in the optimally doped spectra that extends out to ∼1 eV, consistent with a marginal Fermi liquid description of these materials.
In addition, from our measurements of the static component of the charge susceptibility, we have discovered diffuse, short-range charge order in optimally doped Bi2212. Proximity to the charge order state is suspected to be important in unconventional superconductivity, and has previously been seen in many families of high temperature superconductors, including underdoped Bi2212. At underdoping, we observe sharp elastic peaks consistent with previous charge order observations. At optimal doping, we observe diffuse scattering at low temperatures that exhibits a quasielastic broadening. This is a signature of the emergence of fluctuating order on a time scale of 180 fs.
Electron energy loss spectroscopy; Superconductivity; Cuprates
Thu, 09 Mar 2017 00:00:00 GMThttp://hdl.handle.net/2142/976612017-03-09T00:00:00ZVig, Sean D